Need advice on how to gain weight during pregnancy?

I just got back from my doctor visit yesterday and I haven't gained one pound. I'm 19 wks pregnant and my doctor told me I really needed to gain more weight. I thought I was doing everything right with the 3 meals a day and healthy snacks like yogurt, fruit and cheese in between but apparently that's not enough. I'm trying to gain at least 8 pounds before my next monthly checkup. I really need help and any suggestions would be greatly appreciated. Thank you :)